Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-05
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| Document | Type | Active Internet-Draft (individual in sec area) | |
|---|---|---|---|
| Author | Ted Hardie | ||
| Last updated | 2017-02-21 (Latest revision 2017-01-20) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-hardie-privsec-metadata-insertion-05
Network Working Group T. Hardie
Internet-Draft January 20, 2017
Intended status: Informational
Expires: July 24, 2017
Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-05
Abstract
The IAB has published [RFC7624] in response to several revelations of
pervasive attack on Internet communications. This document considers
the implications of protocol designs which associate metadata with
encrypted flows. In particular, it asserts that designs which do so
by explicit actions of the end system are preferable to designs in
which middleboxes insert them.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 24, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Design patterns . . . . . . . . . . . . . . . . . . . . . . . 2
4. Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Deployment considerations . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
To ensure that the Internet can be trusted by users, it is necessary
for the Internet technical community to address the vulnerabilities
exploited in the attacks document in [RFC7258] and the threats
described in [RFC7624]. The goal of this document is to address a
common design pattern which emerges from the increase in encryption:
explicit association of metadata which would previously have been
inferred from the plaintext protocol.
2. Terminology
This document makes extensive use of standard security and privacy
terminology; see [RFC4949] and [RFC6973]. Terms used from [RFC6973]
include Eavesdropper, Observer, Initiator, Intermediary, Recipient,
Attack (in a privacy context), Correlation, Fingerprint, Traffic
Analysis, and Identifiability (and related terms). In addition, we
use terms that are specific to the attacks discussed in [RFC7624].
Terms introduced terms from there include: Pervasive Attack, Passive
Pervasive Attack, Active Pervasive Attack, Observation, Inference,
and Collaborator.
3. Design patterns
One of the core mitigations for the loss of confidentiality in the
presence of pervasive surveillance is data minimization, which limits
the amount of data disclosed to those elements absolutely required to
complete the relevant protocol exchange. When data minimization is
in effect, some information which was previously available may be
removed from specific protocol exchanges. The information may be
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removed explicitly (by a browser suppressing cookies during private
modes, as an example) or by other means. As noted in [RFC7624], some
topologies which aggregate or alter the network path also acted to
reduce the ease with which metadata is available to eavesdroppers.
In some cases, other actors within a protocol context will continue
to have access to the information which has been thus withdrawn from
specific protocol exchanges. If those actors attach the information
as metadata to those protocol exchange, the confidentiality effect of
data minimization is lost.
The restoration of information is particularly tempting for systems
whose primary function is not to provide confidentiality. A proxy
providing compression, for example, may wish to restore the identity
of the requesting party; similarly a VPN system used to provide
channel security may believe that origin IP should be restored.
Actors considering restoring metadata may believe that they
understand the relevant privacy considerations or believe that,
because the primary purpose of the service was not privacy-related,
none exist. Examples of this design pattern include [RFC7239] and
[RFC7871].
4. Advice
Avoid this design pattern. It contributes to the overall loss of
confidentiality for the Internet and trust in the Internet as a
medium. Do not add metadata to flows at intermediary devices unless
a positive affirmation of approval for restoration has been received
from the actor whose data will be added.
Instead, design the protocol so that the actor can add such metadata
themselves so that it flows end-to-end, rather than requiring the
action of other parties. In addition to improving privacy, this
approach ensures consistent availability between the communicating
parties, no matter what path is taken.
As an example, RFC 7871 describes a method that had already been
deployed and notes that it is unlikely that a clean-slate design
would result in this mechanism. If a clean-slate design were to
follow the advice in this document, that design would likely reverse
a core element of RFC 7871: rather than adding metadata at a proxy,
it would provide facilities for end systems to add it to their
initial queries. In the case of RFC 7871, the relevant metadata is
relatively easy for an end system to derive, as STUN [RFC5389]
provides a method for learning the reflexive transport address from
which a client subnet could be derived. By negotiating an EDNS0
option which allowed them to self-populate this data, clients would
be affirming their consent for its use and providing data at a
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granularity with which they were comfortable. This variability would
change the caching behavior for responses from participating servers,
but the same considerations set out in section 7.3.2 and 7.5 apply to
client-supplied subnets as well as they do for proxy supplied
subnets.
From a protocol perspective, in other words, this approach would be a
minor change from RFC 7871, would be as fully featured and would
provide better privacy properties than the opt-in mechanism it
provides. The next section examines why, despite this, deployment
considerations have sometimes trumped cleaner designs.
5. Deployment considerations
There are two common tensions associated with the deployment of
systems which restore metadata. The first is the trade-off in speed
of deployment for different actors. The Forwarded HTTP Extension in
[RFC7239] provides an example of this. When used with a proxy, it
restores information related to the original requesting party, thus
allowing a responding server to tailor responses according to the
original party's region, network, or other characteristics associated
with the identity. It would, of course, be possible for the
originating client to add this data itself, after using STUN
[RFC5389] or a similar mechanism to first determine the information
to declare. This would require, however, full specification and
adoption of this mechanism by the end systems. It would not be
available at all during this period, and would thereafter be limited
to those systems which have been upgraded to include it. The long
tail of browser deployments indicates that many systems might go
without upgrades for a significant period of time. The proxy
infrastructure, in contrast, is commonly under more active management
and represents a much smaller number of elements; this impacts both
the general deployment difficulty and the number of systems which the
origin server must trust.
The second common tension is between the metadata minimization and
the desire to tailor content responses. For origin servers whose
content is common across users, the loss of metadata may have limited
impact on the system's functioning. For other systems, which
commonly tailor content by region or network, the loss of metadata
may imply a loss of functionality. Where the user desires this
functionality, restoration can commonly be achieved by the use of
other identifiers or login procedures. Where the user does not
desire this functionality, but it is a preference of the server or a
third party, adjustment is more difficult. At the extreme, content
blocking by network origin may be a regulatory requirement. Trusting
a network intermediary to provide accurate data is, of course,
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fragile in this case, but it may be a part of the regulatory
framework.
These tensions do not change the basic recommendation, but they
suggest that the parties who are introducing encryption and data
minimization for existing protocols consider carefully whether the
work also implies introducing mechanisms for the end-to-end
provisioning of metadata when a user has actively consented to
provide it.
6. IANA Considerations
This memo makes no request of IANA.
7. Security Considerations
This memorandum describes a design pattern related emerging from
responses to the attacks described in [RFC7258]. Continued use of
this design pattern lowers the impact of mitigations to that attack.
8. Contributors
This document is derived in part from the work initially done on the
Perpass mailing list and at the [STRINT] workshop. It has been
discussed with the IAB's Privacy and Security program, whose review
and input is gratefully acknowledged.
9. References
9.1. Normative References
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
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[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<http://www.rfc-editor.org/info/rfc7624>.
9.2. Informative References
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<http://www.rfc-editor.org/info/rfc7239>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<http://www.rfc-editor.org/info/rfc7871>.
[STRINT] S Farrell, ., "Strint Workshop Report", April 2014,
<https://www.w3.org/2014/strint/draft-iab-strint-
report.html>.
Author's Address
Ted Hardie
Email: ted.ietf@gmail.com
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